Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, do not accept ink. The areas accepting ink are the printing areas and the ink-rejecting areas are the background areas.
Common materials employed for making a lithographic printing material include photographic materials e.g. photosensitive polymer materials or silver salt diffusion transfer reversal (DTR) materials. For instance, in GB 547,795 and GB 891,898 processes for the production of a lithographic printing plate have been described, which comprise exposing to light under a pattern a plate having a hydrophilic base bearing a light-sensitive coating of a material capable of being hardened where exposed to visible light, inducing hardening of the said material in the light-struck areas to form an insoluble resist in such areas, selectively removing the unhardened portions of the coating from the base, applying to the entire surface of the plate an oleophilic--i.e. ink-accepting--film, and selectively removing the resist and the oleophilizing product adhering thereto from the light-struck areas to restore water-receptive, non-printing portions. U.S. Pat. No. 3,260,198 describes the use of a silver layer applied image-wise e.g. by the DTR-process to a hydrophilic layer essentially consisting of at least one metal of the group consisting of aluminium and zinc to protect the underlying hydrophilic layer from being oleophilized, after which the silver image layer is removed by treating the plate with a silver oxidizing agent, thus image-wise uncovering the hydrophilic layer. However, such photographic materials have the disadvantage that they often require strictly controlled ambient conditions before processing and a laborious or time-consuming treatment, and/or that they are ecologically or toxically harmful owing to the use of liquid processing baths. Furthermore, photographic materials that can be developed without the use of liquid processing baths often suffer from the additional disadvantage of being based on chemical compounds that are difficult to prepare.
Heat-sensitive materials recording machine-readable information have been described, in which materials by the thermal action of a high intensity laser beam pits or holes are burnt in a thin metallic film to optically record sound information in digital form. According to a common embodiment the information is stored in digital form on a spinning disk. After the recording a laser beam is used to read out the track of holes as a sequential pattern of light reflection values that are detected electronically. A system based on tellurium as ablatable metal has been described in e.g. Scientific American, August 1980, pages 118-120. The use in optical disk production of a thin layer of bismuth for high density direct read after write (DRAW) recording has been described in Optica Acta, (1977), vol. 24, No. 4, pages 427-431.
Another class of heat-sensitive materials recording human-readable information are e.g. computer output microfilm (COM) materials, the record of which can be read by optical enlargement in a reader upon projecting light through the COM record.
The local removal of a thin metal layer by burning holes has not been restricted to the direct production of optical density or light reflection patterns but has been applied like-wise according to e.g. the published PCT application WO 86/00575 for the production of a stencil that may serve for the production of dye images. According to said PCT application a radiation-sensitive article is provided having at least one vapour-deposited dye layer on the surface of a support and a vapour-deposited, graded metal/metal oxide or metal sulfide layer applied directly over the vapour-deposited dye layer. The dye layer or the metal layer may carry additional layers e.g. vapour-coated organic protective layers. An image can be formed on the graded metal/metal oxide or metal sulfide layers by ablation when struck by heat-generating light such as the light of a high intensity laser beam or of a flash lamp. The holes made in the graded metal/metal oxide or metal sulfide layer by ablation serve as the apertures of a stencil, through which dye can be transferred by heat onto a receptor element.
The image-wise ablation of a thin metal layer by laser light has also been described for the production of a lithographic printing plate. In JP 86046314 a material has been described, which comprises a support, an ink-oil-sensitive layer, and a chromium metal layer. A printing plate is made by directing laser light onto the material and thus removing the chromium metal layer. However, chromium, is known to be a very toxic element and, furthermore, its conductivity makes it less suitable for use as an ablative layer than e.g. bismuth. Moreover, the direct use of the imaging element as a lithographic printing plate may result in short run lengths on a lithographic press since increasing wear of the chromium areas leads to a decrease in hydrophilicity of the background areas.
The image-wise ablation of a metal layer of a heat-sensitive recording material by high intensity laser beam light has also been described in EP-A 489,9721. A dye or dye precursor can be transferred to a receptor element by heat and/or liquid through holes made in said metal layer by laser beam exposure.